Combustion controllers are commonly employed in connection with industrial and commercial boilers for modulating air flow and fuel flow to the burner or burners of the boiler. One type of combustion controller uses parallel positioning of air flow and fuel flow actuators to modulate air flow and fuel flow over the entire operating range of the boiler to ensure the safety, efficiency, and environmental requirements of combustion can be satisfied across the entire operating range. In parallel positioning control systems, the combustion controller controls air flow by manipulating actuators associated with a set of air dampers and/or a variable frequency driver operatively associated with a variable speed air flow fan. The combustion controller also independently controls fuel flow by manipulating fuel actuators, such as solenoid valves or other types of flow servo valves, to increase or decrease fuel flow to match the desired firing rate.
The operating range of a boiler is generally defined by its firing range between a low fire point commensurate with the minimum firing rate at which combustion is sustainable and a high fire point commensurate with the maximum energy output of the burner. The firing range depends on the boiler's burner's turndown ratio, that is, the ratio between the highest energy output and the lowest energy output. For each given firing rate within the boiler firing range a pair of suitable positions of the air supply and fuel supply actuators must be defined. Each pair of actuator positions then corresponds to a defined air/fuel ratio that in turns determines efficiency, emissions and stability of combustion for a resultant firing rate. The determined set of coordinated air and fuel actuator positions provides a map or algorithm that is used by the boiler controller during operation of the boiler to modulate the burner fuel valve and the air damper in response to firing rate.
When a combustion control system is first installed on a boiler, the desired air and fuel actuator positions need to be defined at a number of points, i.e. firing rates, within the firing range, because the relationship between the sets of air and fuel actuator positions to firing rate is non-linear. The process of defining the proper fuel and air actuator positions throughout the firing range is commonly referred to as commissioning of the boiler combustion control system. The purpose of the commissioning process is to find a set of coordinated air and fuel actuator positions at various points, i.e. firing rates, across the operating range such that safety, efficiency, and environmental requirements can be achieved. During the commissioning process, at each of the respective firing rates at which an optimal set coordinated air and fuel actuator positions is determined, the excess oxygen level associated with combustion at those positions is measured and recorded.
Generally, in parallel positioning combustion control, the combustion controller includes a first feedback circuit including a pressure controller for adjusting the firing rate in response to a sensed boiler pressure and a second feedback circuit including an oxygen trim controller for adjusting the excess oxygen level in response to a sensed excess oxygen in the flue gas. Typically, the pressure controller and the oxygen trim controller are of the type commonly referred to PID controllers. Such controllers employ a control function having a proportional term, an integral term and a differential term. In conventional practice, once the commissioning process is completed, it is necessary for the commissioning technician to separately tune the oxygen trim controller and the pressure controller through a trial and error method or step tests. The purpose of the tuning process is to establish the gain factors associated with the proportional, integral and differential terms of the control function to provide a control function that is applicable over the entire firing range of the associated combustion system. The tuning of both controllers after completion of the commissioning process lengthens the time required for a technician to complete installation of the combustion control system.